Publications (23) View all
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Article: In Situ Study of CO Oxidation on HOPG-Supported Pt Nanoparticles.
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ABSTRACT: Catalyzed CO oxidation: A newly designed high-pressure flow cell is utilized to study CO oxidation on Pt nanoparticles in situ by simultaneous quadrupole mass spectrometry and high-pressure X-ray photoelectron spectroscopy. The spectroscopy data show that the active catalyst consists of metallic Pt with both chemisorbed O and patches/shell of surface oxides.ChemPhysChem 03/2013; · 3.41 Impact Factor -
Article: Stability of Pt Nanoparticles Supported on SiO(2)/Si(111): A High Pressure X-ray Photoelectron Spectroscopy Study.
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ABSTRACT: The stability of Pt nanoparticles (NPs) supported on ultrathin SiO2 films on Si(111) was investigated in situ under H2 and O2 (0.5 Torr) by high pressure x-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 °C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtOx desorption from the Pt NPs (~2 nm and ~4 nm NP sizes) at temperatures above 450 °C in the presence of 0.5 Torr of O2. Under oxidizing conditions the entire NPs were oxidized, although with no indication of a PtO2 phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e. minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment.ACS Nano 11/2012; · 10.77 Impact Factor -
Article: Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2.
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ABSTRACT: An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.Journal of the American Chemical Society 08/2011; 133(34):13455-64. · 9.91 Impact Factor -
Article: Oxygen Chemisorption, Formation, and Thermal Stability of Pt Oxides on Pt Nanoparticles Supported on SiO2/Si(001): Size Effects
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ABSTRACT: The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature leads to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 → PtO → Pt. For NPs in the 2–6 nm range, the NP size was found to affect the strength of the O binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10–20% PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2-desorption temperatures for the Pt NPs as compared with the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size.08/2011; -
Article: Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol.
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ABSTRACT: Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.Journal of the American Chemical Society 04/2011; 133(17):6728-35. · 9.91 Impact Factor
